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Showing posts with the label Stormwater Design

Hydraulic Structures: Siphon Spillway Mechanics and Priming Dynamics

 A Siphon Spillway is a closed conduit bent over a dam crest that uses atmospheric pressure differentials to discharge high flows under low operating heads. Flow transitions through three distinct operational phases: ​Weir Flow: Initial rising water level overflows the lower lip as a simple weir. ​Priming Phase: Flow seals the downstream leg outlet, entraining and evacuating internal air to form a partial vacuum within the siphon crown. ​Full Siphonic Flow: Continuous liquid column flow established under total differential head (H). ​The ultimate siphonic discharge (Q) is evaluated using pipe flow hydraulics: $Q = C_d \cdot A \cdot \sqrt{2 \cdot g \cdot H}$ ​Where $C_d$ is discharge coefficient $(\approx 0.6\text{ to }0.8)$ and $A$ is throat cross-sectional area. The maximum operating suction head at the crown is limited by water vapor pressure to prevent air pocket formation and cavitation. ​Siphon spillways installed on medium storage dams across India provide rapid automatic dis...
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Flood Hydrology: Rational Method and Time of Concentration Mechanics

 For small catchments (typically under $50\text{ km}^2),$ the peak surface runoff discharge $(Q_p)$ resulting from a uniform rainfall event is estimated using the Rational Method: $Q_p = 0.278 \cdot C \cdot I \cdot A$ ​Where $Q_p$ is peak flow $(\text{m}^3/\text{s}),$ $C$ is runoff coefficient, $I$ is rainfall intensity $(\text{mm/h}),$ and $A$ is catchment area $(\text{km}^2).$ The critical storm duration occurs when rainfall duration equals the catchment's Time of Concentration $(t_c).$ According to Kirpich’s Equation, $t_c$ (in minutes) is evaluated from physical basin geometry: $t_c = 0.01947 \cdot L^{0.77} \cdot S^{-0.385}$ ​Where $L$ is maximum flow path length (meters) and $S$ is main channel slope $(\text{m/m}).$ ​Applying static runoff coefficients $(C)$ in rapidly urbanizing Indian watersheds leads to significant underestimation of peak discharges, causing urban flash flooding. ​Modern urban hydrology workflows dynamically update C values by overlaying GIS high-resolution...
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